1. Field of the Invention
This invention relates to polymeric compositions which can be crosslinked to produce heat resistant and flame resistant products useful in the production of coated wire and cable products as well as film sheet and molded products. More particularly, the invention relates to flame retardant crosslinkable ethylene-vinyl ester and ethylene-alkyl acrylate copolymer compositions which exhibit improved extrusion processability and, after crosslinking, improved resistance to degradation by heat aging.
2. Description of the Prior Art
Fire resistant polymer compositions are widely utilized for wire and cable insulation. In electrical environments both insulating and fire resistant properties are considered to be necessary. For flame resistance, extrudable compositions available to the wire and cable art were at one time required to contain sizeable quantities of halogenated polymers, such as chlorinated polyethylene, polyvinyl chloride, chlorinated polybutadiene, chlorinated paraffin, etc., and antimony trioxide. Alternatively, a coating of chlorosulfonated polyethylene paint was applied to a nonflame retardant insulating compound which required an additional manufacturing operation.
In certain applications electrical failures occurred due to migration of the organic insulating component. The problem was solved through the addition of hydrated alumina to compositions whose organic binder consisted of butyl rubber, epoxy resins or polyester resins. Such compositions are disclosed in Kessel et al U.S. Pat. Nos. 2,997,526, 2,997,527, and 2,997,528. The compositions, however, did not have an acceptable balance of processability and extrudability characteristics, physical and electrical properties, heat resistance and flame resistance. Furthermore, these compositions exhibited unacceptable tensile strength, elongation and percent elongation after aging.
Fire retarding polymeric compositions exhibiting improved moisture and heat resistance comprised of a crosslinkable polymer, such as ethylene-vinyl acetate copolymer, one or more silanes and one or more hydrated inorganic fillers have found wide acceptance in the wire and cable industry. Such compositions are disclosed in U.S. Pat. Nos. 3,832,326 and 3,922,442 to North et al and U.S. Pat. Nos. 4,349,605 and 4,381,362 to Biggs et al. Additives such as pigments, stabilizers, lubricants, and antioxidants are also typically included with the crosslinkable polymer, silane and hydrated filler. Such formulated compositions exhibit a unique balance of processability, physical and electrical properties, and a high degree of flame and fire retardance. Moreover, these highly desirable results are achieved (a) without the use of halogenated polymers, such as polyvinyl chloride and chlorosulfonated polyethylene, thereby eliminating potential for generating dangerous hydrogen chloride fumes; (b) without the use of carbon black thereby making it possible to formulate colored insulations; (c) without the application of any flame retardant coatings thereby eliminating the need for an additional step in manufacturing operations after the insulating compound is extruded onto the conductor; and (d) without the use of antimony trioxide thereby eliminating the need to use a substantial quantity of an expensive compounding ingredient.
The compositions of North et. al., and Biggs et. al., find particular utility where white and colored insulation compositions are extruded over metal conductors, e.g., copper or aluminum wire, to provide a single layer insulating and jacketing composition which meets the automotive primary SAE J1128 standards and UL 125.degree. C. appliance wire SIS standards. These are widely used for switchboard wire, appliance wire and automotive wire where a unique combination of superior electrical properties combined with resistance to the degradative effects of heat and flame are essential and where low smoke density and non-corrosive fumes are desirable.
The use of petroleum oils in the compounding of rubbers is widely practiced. Many elastomers do not develop crystallinity upon stretching and exhibit relatively low gum tensile strengths so that reinforcement by carbon black and other fillers is necessary to develop the strength demanded of rubber products. Petroleum oils function to provide the capability for high loading of reinforcing filler to obtain the desired physical properties while improving processability for mixing and fabrication of a rubber composition. Processing improvements can include reducing the viscosity of the formulation, facilitating dispersion of fillers and pigments, reducing mixing temperature and power requirements and providing the necessary flow characteristics to fabricate and cure the composition in a final product form. Oils can also reduce shrinkage and modify the performance properties of the resulting vulcanizate. While the amount of the petroleum processing oils can vary, depending on the type of oil, the elastomer being formulated and the filler level, they are typically utilized in amounts from about 10 parts up to as high as 150 parts per 100 parts elastomer. Most commonly, the oils are utilized from about 25 to 50 parts per hundred per elastomer. Petroleum oils are typically not utilized with polyolefin type resins since they have very limited compatibility with polyolefin polymers, provide little plasticization and have a tendency to exude to the polymer surface with aging. In contrast to many elastomers, many polyolefin polymer do develop crystallinity with stretching and the addition of a petroleum oil can detract from reinforcement by a filler. At the concentration levels employed with elastomers, petroleum oils can detract from processability and compound preparation. The oils can cause slippage to prevent development of adequate shear during compound mixing. Also, in the fabrication of the compound, petroleum oils can function as an external lubricant and prevent the homogeneous processing of the compound to a final product form and cause unsteady processing conditions.
We have now unexpectedly discovered that by incorporating low levels of certain hydrocarbon processing oils into flame retardant crosslinkable polyolefin compositions improved processability and physical properties can be realized. It has also quite unexpectedly been discovered that the addition of small amounts of these petroleum oils to ethylene copolymers significantly improves the resistance to scorch and substantially lowers processing power requirements and pressure development.
It is an object of this invention to provide improved crosslinkable flame retardant polymeric compositions based on ethylene-vinyl ester copolymers and ethylene-alkyl acrylate copolymers which exhibit superior processing characteristics and have improved resistance to scorch. It is a further object to provide crosslinked compositions which exhibit improved resistance to oxidative degradation.